Design of Zinc Oxide
Tetrapod Devices
In fulfillment of the requirements of
ENMA490: Materials Design
Spring 2007
University of Maryland College Park
From Left: Michael Figueroa, Abiodun Osho, ,Yilin Liu, Arthur M. Grace, Matthew Castille,
Margaret Bennett, Patrick Stahl, Aron Cepler, Paul Pineau
Not Pictured: Brian Smith
Special Thanks to Professor Gary Rubloff and Parag Banerjee for their guidance and technical support.
Thanks also to Susan Beatty, Dr. Tim Zhang, Laurent Henn-Lecordier Tom Loughran, Cytimmune, and
Northrop Grumman.
Work completed at Materials Teaching Lab, FabLab, LAMP Lab, and NISP Lab.
Outline
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Motivation
Logistics
Timeline
Fabrication of Nanostructures
Different Device Concepts
Integration and Testing of Devices
Lessons Learned
Future Work
Summary
Motivation
• The unique characteristics of ZnO nanomaterials
(wide band gap, piezoelectric effect) lends them
great potential for UV and pressure sensing
applications.
• Nanomaterials require precise structural and
electrical characterization which are complicated
by their size.
• In order to use ZnO nanostructures successfully
in macro-scale devices, we must develop
effective means to integrate nanostructures into
a working device.
Logistics
• The ENMA 490 class was divided into 5 different
groups: Research, Management/Writing, Synthesis,
Device Design, Integration/Application
– Research: Use scholarly articles to understand properties of
ZnO nanoparticles
– Management/Writing: Guide all groups in the right direction;
major writer of report and presentation
– Synthesis: Making nanoparticles
– Device design/fabrication: Work out physical problems with
device (i.e. attaching nanoparticles to substrate)
– Integration/Application: Testing of devices produced
• Group membership was not static; many members
moved around to different groups as needed
Milestones
2/12
First Nano
structures
2/26
Made
Electrode
mask
3/12
Finish Mid term
presentation
SEM of Gold
Nano particles
3/12
Take SEM of
First of first
Finish Mid
term
presentation
4/2
Made 6 PDMS
Test First Device
Testing of 6
PDMS device
Spring Break
4/16
Made PDMS
covered
device
4/16
Test
PDMS
devices
4/23
Made quantitative testing
device, PVA, Kapton, PVA
devices
4/9
Made
Kapton
tape
device
Synthesis of the Nanostructures
Used Vapor Phase Transport Method
Furnace
Valves
O2
Ar
Glass Tube
Gas Flow
Alumina Boat
• Argon and Oxygen gas flow
• Bubbles per minute controlled the rate
• Inside the tube, solid reagents were
placed in the boat.
• In some runs, Si wafers with gold
catalyst were placed downstream from
the boat.
Processing Condition A: No Wafer
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Solid Reagents: Zn metal powder
Gas Flow: Ar- 30 bubbles/min
O2- 10 bubbles/min
850 Celsius and held for 90 minutes.
Initially only Ar gas, then turn on O2 gas
when system reached 410 degrees.
Results show a fluffy white product
throughout much of the tube.
See Zinc Oxide Tetrapod Structures
(ZOT) from ESEM images:
Scale Bar: 20μm
Scale Bar: 5μm
Processing Condition B: Colloidal Gold
•Reasoning: Instead of Zn and O2 reacting
in air (or on the glass tube/ceramic boat) to
form tetrapods, a catalyst-induced
nucleation may cause linear rods to grow.
Si Wafer, Scale Bar: 1μm
•Cytimmune: 26 nm in 133 μg/ml H20
•Applied colloid to Si and GaN wafers.
•Placed Zn powder in boat and wafers
downstream from boat. Same gas flow and
temperature conditions as Experiment A
GaN Wafer, Scale Bar: 1μm
Processing Condition C:
ZnO and Graphite with Colloidal Gold
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ZnO and graphite source material
instead of Zn powder.
Use 1:1 ratio of ZnO and graphite
powder. Only use Argon for gas flow.
Heat to 900 degrees and dwell for 90
minutes.
Mechanism: ZnO powder is reduced
by graphite to form Zn and CO vapor at
high T (carbothermal reduction).
Scale Bar: 2 μm
Zn vapor flows downstream to form
alloy with Au colloid.
Vapor-Liquid-Solid growth: ZnO
nanowires form, possibly from Zn and
CO reaction.
Tube setup from left: GaN, Si, Boat with mixed 1:1 ratio of ZnO and graphite
Processing Condition D: Gold Coated Wafers
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Motivation: a uniform 10 nm goldcoated Si wafer will induce better
nucleation sites for nanorod growth
than colloidal gold
Used ZnO and C powder with Ar and
O2 gas
Heated to 900 Celsius
See rod-like structures, forming more
material than with gold colloid
processing conditions.
Scale Bar: 10μm
Conclusions from nanostructure synthesis:
•Processing Condition A (without wafer)
produced high yield of ZnO tetrapods
(ZOTs).
•Methods employing wafers with gold
colloid or coating were able to grow
nanorods, but not in high enough yield to
collect and use in devices.
•All devices used ZOTs fabricated from
processing condition A.
Horizontal and Vertical Device Concepts
Horizontal Device Top View
Vertical Device Cross Section
Figure A
Figure B
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In Figure A, the unaligned
nanostructures are placed
between two electrodes.
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In Figure B, a pattern is used to
induce alignment in rod-like
nanomaterial.
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In both figures, the light blue area
indicates nanomaterial, yellow
block show the polymer matrix,
and dark blue shapes represent
the electrodes
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One concept involved use of a
patterned catalyst (red) to grow
vertically aligned nanorods.
Horizontal Device Processing
Photoresist
Active Area
Trenches
Contact Holes
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Au Electrode
Multiple electrode variations were fabricated
Common features include a protective layer of photoresist to cover the electrodes and two holes in
the layer to provide contact points
One version had trenches between the electrodes. If a polymer-dispersed method was used, the
trenches were intended to keep the liquid dispersed nanomaterial in place.
Another version has one open field between the electrodes.
Two masks were used to create each electrode design
Integrating Nanomaterial into
Device
• Langmuir-Blodgett and integration via PDMS fluidic flows
were investigated. These techniques were considered
too complex for our time frame.
• Polymer matrix dispersions chosen
• Various polymers were researched and utilized in
experiments
Kapton
www2.dupont.com/Kapton/en_US/assets/d
ownloads/pdf/summaryofprop.pdf
PDMS
Jung, et al. Nano Letters,
Vol.,6, No. 3 413-418. 2006
PVA Glue
Our First Device
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Two silver paint contacts on
a microscope slide ~½ inch
apart.
A pile of ZOT was placed in
between (see A).
More silver paint was
applied at the contact
regions (see B).
Scale Bar: 20μm
An ESEM image of the Ag contact / ZOT interface.
Scale Bar: 5μm
A
B
A higher magnification of the Ag / ZOT interface. The arrow
indicates a ZOT arm clearly embedded in the contact metal..
Proof of Concept: Initial Device Testing
• The device was tested on
the probe station in the
Kim Building teaching lab.
• Force was applied by
hand with a glass rod
laying across the
nanoparticles.
• Light was applied with a
lamp at the work station
• From both tests we saw
electrical response and
decided to investigate
pressure applications of
ZnO nanostructures
Considerations: Pressure Device
• Before we integrated the ZOT into our microdevice we
needed to test several approaches by making macrodevices
using:
– Kapton Tape
– PDMS
– Poly Vinyl Acetate Glue
• The qualitative results obtained from these macrodevices
would allow us to select the best method of integration into
our microdevices.
• Considerations we wanted to address in our macrodevice
were:
– ZOT connectivity in the given integration method
– Noticeable electrical response to stimuli
– Structural Stability of the device
– Ease of integration into microdevice
Zinc Oxide Tetrapods (ZOT) in PDMS
• For our pressure sensing device,
we investigated two devices that
used PDMS. The first device
contained a mixture of PDMS and
the ZOT powder
The IV curve of 29 wt. % ZOT shows a lack of
repeatability. It was determined through ESEM
images that there was little connectivity between
the dispersed ZOT at this wt.% High wt.% designs
were too mechanically unstable to test.
29 wt. % ZOT suspended in PDMS
PDMS Cover Design
– When we reversed the
polarities, the results were not
reproducible
– There was a noticeable
response to pressure
– These unique characteristics
were too complex for our
timeframe
ZOT powder covered with PDMS
IV C urve P DMS C over on Z OT
6.00E -09
5.00E -09
4.00E -09
3.00E -09
C urrent (Am ps )
• Pile of ZOT (similar to device
one) covered with PDMS and
degassed.
• When we reversed the device,
unique characteristics were
observed as shown in the
curve
• From this device we concluded
that:
2.00E -09
1.00E -09 0
0.00E + 00
-15
-10
-5
0
5
10
-1.00E -09
-2.00E -09
-3.00E -09
Volta g e (v)
Tes t 1
Tes t 3 (s ec ond revers al tes t)
Tes t 2 (R evers al)
Tes t 4 (40.996 grams )
15
Kapton Tape over Nanorods
• For our second device we decided to essentially bundle the
nanostructures with Kapton tape
• From our tests we concluded that there is an electrical
response with the application of pressure within the device
but integration of the Kapton tape into a microdevice would
be very difficult
IV Curves for Kapton Tape
8.00E-10
6.00E-10
Current (Amps)
4.00E-10
2.00E-10
Applied Force Test 1
-6
-4
0.00E+00
-2
0
-2.00E-10
-4.00E-10
-6.00E-10
-8.00E-10
Voltage (V)
Applied Force Test 2
2
4
6
No Force Applied
PVA Adhesive and ZOT
IV Curves for Glue Pressure Tests
3.00E-07
2.50E-07
2.00E-07
Current (Amps)
• Neutral pH adhesive (polyvinyl
acetate based; PVA) was
diluted with water
• Integration Methods:
– dripping diluted glue onto
the powder in place
– mixing a slurry of glue,
water and ZOT and
depositing this on the
substrate.
• From this we concluded:
– The nanorods had an
electrical response with the
application of pressure
– There was relative ease in
making the device
– That we had created a
structurally stable structure
1.50E-07
-6
-4
1.00E-07
No Pressure
5.00E-08
First Pressure Test
0.00E+00
-2
0
-5.00E-08
Second Pressure Test
-1.00E-07
-1.50E-07
-2.00E-07
Voltage (V)
2
4
6
Device Selection
• From the qualitative data we obtained from
the macrodevices, we decided to further
our venture into a microdevice using PVA
glue as an integration staple because:
– We felt it would be easiest to integrate
– There was discernable electrical response
(which allowed us to assume there was good
conductivity)
– The structure of the device was stable
Quantifying Pressure Sensing
• In order to adequately access whether the conductance/resistance
is changing with pressure we made a special platform that allowed
us to apply quantifiable pressure on top of a device.
• What we expect is that the conductance should decrease with the
application of pressure
Apparatus created to apply quantifiable
pressure on device. Rubber tip diameter is
3mm
Pressure Testing: Glue Dispersed Powder
on the Au Patterned Electrodes
Pressure on Glue-Dispersed ZnO - Trial 1
3.00E-05
2.00E-05
Increasing Pressure
Amps
1.00E-05
-4.5
-3.5
-2.5
-1.5
0.00E+00
-0.5
-1.00E-05
Increasing Pressure
0.5
1.5
2.5
3.5
4.5
1kPa
3kPa
4.5kPa
6kPa
9kPa
15kPa
-2.00E-05
-3.00E-05
Volts
As the applied pressure increases, a noticeable decrease in the slope is shown.
Pressure Testing: Glue Dispersed Powder
on the Au Patterned Electrodes
Pressure on Glue-Dispersed ZnO
2.40E+05
2.20E+05
Resistance (ohms)
2.00E+05
1kN
1.80E+05
Increasing Pressure
3kN
Increasing Resistance
4.5kN
6kN
1.60E+05
9kN
1.40E+05
15kN
1.20E+05
-4.5
-3.5
-2.5
-1.5
1.00E+05
-0.5
0.5
1.5
2.5
3.5
4.5
Volts
Resistance varies with pressure between 1.5 and 3.5 V in both positive and negative
regions.
Lessons Learned
• Working in a group of 10 is challenging:
– Organization
– Communication
– Scheduling
• Even though BlackBoard is a good tool,
individual lab notebooks would have been useful
• Competition for resources/equipment is fierce
• Planning and executing a multi-phased project
requires technical foresight.
Future Work
• Better apparatus for applying quantifiable
pressure
• Worth quantitatively investigating capacitance as
a function of voltage
• Comparing electrical and piezoelectric response
of nanorods versus tetrapods
• Modeling pressure versus resistance from data
we obtain is necessary in order to:
– Make a predictable device
– Understand the nanomechanics of ZOT PVA
composite
Summary
• Synthesized nanorods and tetrapods using vapor
transport growth
• Used lithography to create two horizontal electrode
designs on silicon wafers
• Integrated tetrapods into macroscale devices with:
– Kapton Tape
– PDMS
– PVA Glue
• Observed I-V response to pressure and light stimuli in
devices
• Quantified pressure effects on resistance on a wafer
device